Conditional subscribing/un-subscribing from multiple messages

Question

This is a follow-on question to Is publishing/subscribing to events after UI rendering a best practice regardless of framework? which I asked last month as we first went down the pub/sub path with a Javascript/ExtJS application. If you want to get to the meat of my question from a purely publish-subscribe standpoint, please see the first, emphasized, sentence of the next to last paragraph. First a bit of context:

We have a panel that contains a tab-panel whose cards hold a grid panel, and a panel that holds a form for filtering the grid data. In the beforechangetab event we add/remove a shared grid and publish GRID_TAB_TOGGLED message along with a flag indicating stocked or non-stocked, and the grid then reloads with the appropriate data.

As for the filters, the code I inherited relied on just one form, and combo box choices get modified based on the loaded stocked/non-stocked data, but now we are required to retain all of the filter state. My initial idea is to use a card layout for holding either stocked or non-stocked filters, and toggling between these in response to the same GRID_TAB_TOGGLED message.

The issue though is that both of these filter forms need to listen for other, numerous, identical events. I think I've determined a solution, and am seeking design validation.

My idea is for the different forms to be able to determine whether they are the "active" (stocked or non-stocked) filters, and if active, subscribe to all necessary messages, but if not active, unsubscribe from them all. This un-subscribing, re-subscribing then would occur whenever the GRID_TAB_TOGGLE message was received.

Does this seem like a reasonable design, from an ExtJS standpoint, but of more interest to me (since I have nearly 3 years ExtJS experience, but only 1.5 months implementing my own pub/sub based system), from a publish/subscribe paradigm standpoint? Perhaps this is even a pattern (or, though I hope not, an anti-pattern) for pub/sub based systems?

Rene Saarsoo · Accepted Answer · 2012-02-20T05:46:28.290

I would avoid excessive subscribing/unsubscribing. If you are completely removing a component, then it's obvious that you should unsubscribe it from all the events. But when you are just disabling the component temporarily, then you should also just disable the events temporarily.

So, instead of unsubscribing from all the events and then re-subscribing to them again I would use some kind of mediator that would forward me all the events and would allow to easily suspend all its events. This should be easy to implement using #suspendEvents and #resumeEvents of Ext.util.Observable.

Ext.define("EventMediator", {
    extend: "Ext.util.Observable",
    constructor: function(cmp, events) {
        this.relayEvents(cmp, events);
        this.callParent([]);
    }
});

var med = new EventMediator(grid, ["click", "dblclick"]);

med.on("click", form.doSomething);
med.on("dblclick", form.doSomethingElse);

// When form gets hidden:
med.suspendEvents();

Conditional subscribing/un-subscribing from multiple messages

1 Answers1